1. Field of the Invention
The present invention relates to a method of determining the endpoint of a chemical mechanical polishing (CMP) process, and more particularly, to a method of CMP endpoint detection involving the use of infrared spectroscopy.
2. Description of the Prior Art
Chemical mechanical polishing is a common method used in the semiconductor industry to planarize the surface of a semiconductor wafer. For example, it can be used to remove a first layer of a dual layer surface. Several methods are available for determining the endpoint of the CMP process, with the most common being optically monitoring a target layer. However, the target layer is required to be of a sufficient thickness so that during the CMP process, data can be detected by the photo detector of the intensity of a reflected light beam to produce a trace curve of which is then used to determine the CMP endpoint. Generally, the thickness of the target layer is required to be greater than 3000 angstroms so that the data generated by the reflecting light beam produces a trace curve.
Please refer to FIG. 1 of the schematic diagram of the method used to determine the CMP endpoint according to the prior art. As shown in FIG. 1, an unpolished semiconductor wafer 11 is positioned within a holder 13 of a wafer head 15. Beneath the wafer 11 is a polishing pad 12 supported by a polishing platen 16, with a window (not shown) penetrating both the pad 12 and the platen 16 to the surface of the target layer of the semiconductor wafer 11. A motor 19 drives both the wafer head 15 and the polishing platen 16, while a controller 18 controls both their rotational speeds. A vertical motor 20 is positioned for the vertical contacts between the wafer head 15 and the polishing platen 16. In addition, the equipment of the CMP process also includes a slurry supplier tube 14, to transfer a flow of slurry between the semiconductor wafer 11 and the polishing pad 12.
During the CMP process, the wafer head 15 and the polishing platen 16 both rotate, respectively, at a specified rate of speed to allow the slurry to smoothly spray the polishing pad 12. With the proper parameter settings, the target layer of the semiconductor wafer 11 can be polished via the chemical reaction produced between the slurry and the mechanical polishing of the polishing pad 12. The CMP endpoint detecting system of the prior art determines the polishing endpoint by a trace curve 22, processed by a computer 21, of the light beam reflected from the target layer. More specifically, the equipment of the CMP process of the prior art includes an optical detecting device 17 to generate a light beam of a specific wavenumber. The light beam passes through the hole of the polishing pad 12 and is directed onto the target layer of the semiconductor wafer 11 at a predetermined angle. The intensity of the reflected light beam can be continually detected by the optical detecting device 17. Then, the data is transmitted to the controller 18 and the computer 21 where the result is shown as a trace curve 22 on the computer screen. From the trace curve 22, the CMP endpoint is then determined by the use of predetermined window logics 51 and 52 during abrupt changes in the intensity I of the reflected light.
However, the prior art method of determining the endpoint of the CMP process requires a target layer of a thickness above 3000 angstroms in order to produce a computer-generated trace curve, which is then used to detect the CMP endpoint.
It is therefore a primary objective of the present invention to provide a novel method of CMP endpoint detection without the need for a specific target layer thickness.
In a preferred embodiment, the present invention provides an infrared spectroscopic method of removing a first layer from a semiconductor wafer without overpolishing the underlying second layer. The first layer and the second layer of the semiconductor wafer are composed of silicon oxide or silicon nitride. An infrared (IR) light source is directed onto the semiconductor wafer, and data related to IR absorptivity of each layer is collected to produce a standard IR absorbance curve for each layer of the semiconductor wafer. Since each layer absorbs IR light at different wavenumbers, two defined IR absorbance curves are observed whereby once the CMP process is performed, a change in the IR absorptivity and thus the absorbance curve of each layer is detected. The IR absorptivity of the first layer progressively decreases for a length of time until significant change in the absorbance curve is no longer detected. The endpoint of the CMP process is determined at a point when significant change in the IR absorptivity of the first layer is no longer detected and change in the IR absorptivity of the second layer occurs.
It is an advantage of the present invention that the endpoint of the CMP process is easily and precisely determined via infrared spectroscopy, whereby removal of a first layer exposes, but does not overpolish, the underlying second layer of a semiconductor wafer.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.